Plastic solder array using injection molded solder

ABSTRACT

Disclosed is a solder injection mold apparatus and method for providing solder balls to a printed circuit board substrate using the solder injection mold apparatus in the plastic ball grid array (PBGA). The solder mold through holes are chamfered at entry and at exit ends to assist in receipt of molten solder and the formation and transfer of solder balls to lands on the substrate. A blind recess is provided in the second major surface of the mold, i.e. the side facing the substrate, in order to accommodate electronic components mounted thereon. Solder balls are delivered and metallurgically affixed to the lands in a process that requires only one reflow, leaving the through holes clean of solder and the mold ready for reuse. The material of which the substrate, mold and base plate are comprised is selected to be non-wettable by solder and mutually compatible with respect to CTE when exposed to temperatures of molten solder.

This is a division of application Ser. No. 09/067,904, filed Apr. 27,1998, now U.S. Pat. No. 6,029,882.

FIELD OF THE INVENTION

This invention relates to the field of electronic packaging. Moreparticularly, this invention relates to the packaging of integratedcircuits (ICs) and mounting the ICs onto laminated printed circuitboards (PCBs) by means of solder balls using a technique known asplastic ball grid array (PBGA).

BACKGROUND OF THE INVENTION

ICs comprise a complex assortment of microminiature electroniccomponents on a silicon chip. To protect them from damage due tohandling and moisture, ICs are typically housed in a ceramic or plasticpackage that is either soldered to a PCB or inserted into a matchingsocket which has been soldered into a printed circuit board. For IChousing, ceramic is the material favored in high reliability industrialand military applications, plastic, being less costly, is favored incommercial and consumer products. Many millions of these types ofstructures are manufactured every year, worldwide. For the PCBsubstrate, FR4 resin laminate is becoming an industry norm because ofits electrical, chemical and physical properties and its low cost.

Means by which the IC package has been attached electronically to a PCBor another IC include metal pins in plastic pin grid array (PPGA), silkscreened solder paste, solder preforms and solder balls. Injectionmolded techniques have been used in some cases to apply solder. In PBGA,solder balls are heated to a temperature at which it will reflow, andthe solder is applied to contact pads on the PCB, also referred toherein as lands, electrically connecting the IC to the PCB. Some of theplastic packages have material sculpted out from the middle to form arecess in which the IC die will be placed when the plastic parts arelaminated together. After the IC die is installed in the package,electrical connections are made to the die contact pads and to solderthrough the conductive metal patterns within the package. Thesesoldering processes may involve two reflow heating steps, the firstreflow to attach solder at selected peripheral points on the surface ofthe laminate, and the second reflow to reshape the solder to a sphere.

U.S. Pat. No. 4,585,157 to Stephen R. Belcher describes a method ofbonding two IC chips face-to-face in a type of lead frame wire bondingcalled tape automated bonding (TAB). Rather than testing a chip at asite on the lead frame, the patented method uses a tape bonding approachwherein the finger leads at a single site in a lead frame are dividedinto two groups and are bonded in two separate compression bondingprocesses. In the second bonding process, the first chip is flippedupside down and fit into a recess in a second thermode. The presentinvention is not directed to TAB, but rather is directed to a means andprocess of delivering discrete amounts of solder in PBGA to connectionlands on a PCB having an IC mounted thereon.

U.S. Pat. No. 5,979,664 to Alan M. Lyons et al describes a method oftreating solder used in mounting an IC to a PC board in order toeliminate oxide at the solder surface without flux. To remove the oxidewithout flux, nitrogen or argon gas is flowed over the heated areasbeing joined. Unlike the present invention, the patent is not directedto the solder delivery itself, and in particular not directed to asolder injection mold delivery system and process.

U.S. Pat. No. 5,244,143 to Ference et al, assigned to the same assigneeas the present invention, describes various embodiments of injectionmold apparatus for solder. In the present invention, an injection moldapparatus for use with PBGA is described and a process set forth whichis particularly adapted to PBGA. The patent is useful for itsdescription of the solder reservoir.

U.S. Pat. No. 5,551,148 to Kazui et al describes a flexible film havingtapered through-holes filled with solder, which is heated andtransferred under pressure from the larger diameter end of the throughholes to a circuit board as bumps on pads. The transfer of solder is nogood if the pressure is too low. At that point the flexible film isremoved either by dissolving in hydrazine or ethylene diamine,sublimated by the heat of the soldering process, or peeled away. Thesolder injection mold of the present invention is not a flexible film,but a sturdy, reusable transfer device. Since it does not have to bedissolved, sublimated or peeled away, there is no handling of harshchemicals or destruction or distortion of the injection mold used in thepresent invention.

None of the above references addresses the problems, achieves theresults or discloses the process and apparatus of the present invention.

SUMMARY OF THE INVENTION

Accordingly it is an object of the invention to provide a solderinjection mold and process which is particularly suitable for supplyingsolder to a PBGA structure.

It is a further object of the invention to provide a solder injectionmold which will receive molten solder into a number of through-holesopening at a first major surface and dispense solder balls from thosethrough-holes opening at a second major surface onto and in alignmentwith contact lands on the first major surface of a substrate, inparticular onto a substrate having at least one electronic componentmounted thereon, in a controlled and reliable fashion. It is a furtherobject of the invention to maintain the solder in place in the throughholes within the mold through reflow and cooling of the molten solderinto balls, and to then remove the mold from the substrate, leaving thesolder balls behind and attached in ohmic contact to their respectivelands on the substrate. Still another object of the invention is toprovide a method of attaching solder balls to laminate carriers in aPBGA process with only one furnace reflow using IMS technology. It isalso an object of the invention to avoid damage or destruction to anysubstrate, component, base plate or mold and to reuse the mold and baseplate in the processing of a large number of substrates.

These and other objects are accomplished in the present invention usingan injection mold which includes on its second major surface a blindrecess and at its first major surface a head, or reservoir, for deliveryof molten solder to an array of through-holes in the mold which aredouble chamfered, i.e. fluted, with the wide ends facing outward at bothends of a through-hole which is shaped somewhat like an hour glass, abase plate for containing the molten solder within the through-holesuntil solder balls are formed and attached to a substrate, and asubstrate for receiving the solder balls at their respective contactlands.

Because of the uniqueness of the shape of the through-hole, moltensolder flows into the upper chamfer, filling the through-hole, and isreleased from the lower chamfer to the contact land of the substrate asa solder ball during reflow. The combination of non-wettability of themold material, the double chamfers and the aspect ratio of the moldthrough-holes permit the solder balls to form and release cleanly duringreflow. Only one reflow step is required. The base plate and theinjection mold can be made of the same material as the PCB substrate, orof a different non-wettable material having a substantially similarcoefficient of thermal expansion (CTE). The non-wettability of the baseplate material helps assure that the solder will form a ball and notstick to the base plate. The material of which the mold is made must benon-wettable so that the solder balls will form and adhere to thesubstrate without leaving solder residue within the walls of the moldthrough-holes. A clean mold can be reused easily. The mold material inthe present invention has a life of about 10,000 usages and iscompatible with the temperature of the molten eutectic solder. The CTEcompatibility assures that solder balls in the mold through-holes willtransfer to the PCB in the places desired, and that the mold materialwill not crack during solder transfer. It may be necessary to applypressure between the mold and the substrate during reflow if, as aresult of the material chosen, there is slight deviation from absoluteflatness of these two parts with respect to each other at thetemperature of the molten solder.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a drawing, seen from the top, of the top piece of the PBGAmold, showing the through holes to be filled with solder.

FIG. 1B is a drawing, seen from the bottom, of the top piece of the PBGAmold, showing the through holes to be filled with solder and a blindrecess at the center.

FIG. 2A is a cross-sectional side view showing the profile ofsolder-filled through-holes of the PBGA mold prior to reflow;

FIG. 2B is a cross-sectional side view showing the profile of athrough-hole of the PBGA mold after reflow.

FIGS. 3A-3F is a series of cross-sectional side view drawings showing 6key steps in the PBGA injection molded soldering process.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In order to facilitate understanding of the present invention, referenceis made to the following detailed description taken in conjunction withthe drawings:

FIG. 1A is a view of the first major surface of PBGA injection mold 1with first openings of through-holes 2 arranged in a peripheral array.The aspect ratio (depth:unchamfered width) of the hole is 1:1 or less,preferentially about 3:4 to about 1:2, a suitable width at center beingabout 0.035 inches. The solder is inserted into the mold at this end ofthrough-holes 2 from a reservoir of molten solder, shown in FIG. 3 asfeature 5, which is presented in more detail in U.S. Pat. No. 5,244,143.The temperature of the molten solder can be monitored in the reservoirfor quality control. The mold is preferably comprised of graphite or FR4resin laminate so that it has a (CTE) the same as, or substantiallysimilar to, that of the PCB which is to receive the reflowed solder.Compatible CTEs will assure that the through holes containing thereflowed solder will attain alignment with the lands in the PCB thatwill receive the solder balls and that the mold itself will not crack atthe heated solder temperature. FIG. 1B is a view of the second majorsurface of the PBGA injection mold 1 showing the second openings of moldthrough-holes 2, i.e. ends opposite to those seen in 1A. At the endsseen in FIG. 1B, solder balls 8 (not shown) will emerge after reflow andcooling to join to lands 10 on the substrate 6 (all shown in FIG. 2B).Blind recessed area 3 shown at the center of FIG. 1B is shaped toaccommodate, with clearance, IC chip 7 (shown in FIG. 3E) mounted on PCBsubstrate 6 during solder transfer from the injection mold 1 to the PCBlands 10.

FIG. 2A is a cross sectional side view of PBGA injection mold 1 showingthrough-hole 2 filled with solder 9 before reflow. Through-hole 2includes chamfers 12, shown also in FIG. 2B, at both ends to facilitatereceiving solder 9 in molten form at a first end and to facilitate therelease of solder ball 8 (shown in FIG. 2B) at a second end afterreflow. FIG. 2B shows the solder ball 8 attached to contact land 10 onsubstrate 6 after reflow and prior to removal of mold 1. In FIG. 2B, thesolder 9 seen in FIG. 2A has retracted from the chamfered ends of thethrough-hole into the ball shape during reflow due to surface tensionand has become attached to contact land 10 to which it is aligned.

FIG. 3 illustrates the overall process of PGBA solder injection moldingin six cross-sectional side views. Although the illustrations A-F do notshow it, all through holes 2 are chamfered as shown in FIG. 2. In FIG.3A are seen the injection mold 1, the unfilled through holes 2, and theblind recess 3 to create overhead clearance for an IC chip 7 and anyother components as seen in FIG. 3D. In FIG. 3B are seen injection mold1 in position atop a temporary base plate 4. Base plate 4 must be ableto withstand exposure to the temperature of molten solder withoutdistorting, reacting chemically or being wet by the molten solder. IMShead 5, which is filled with eutectic solder (63%:37% Sn:Pb, reflow @183degrees C.), is slideably mounted on the mold and is seen in motiontoward the left with respect to the mold, having filled sixthrough-holes 2 with solder, indicated as darker through-holes 9. Othersolder compositions may be used, provided the properties of thecomposition selected are compatible with, and nondestructive of, theother materials in the PBGA. In FIG. 3C all through-holes 2 have beensolder filled and the temporary base plate 4, which had been put inplace in 3B to contain the molten solder 9 during filling of the throughholes 2 from IMS head 5, has been removed, since the solder 9 is nowsolid within the chamfered through-holes 2. The substrate 6 may betreated with a small amount of flux, such as a water soluble flux or "noclean" flux, depending on whether the gaseous environment duringtransfer is oxidizing or not, e.g. whether it is air or nitrogen. Mold 1is now ready to be lowered onto the top surface of a substrate 6, whichhas occurred in FIG. 3D. In the FIG. 3D example, blind recess 3 iscovering, with clearance, two chips 7 mounted on substrate 6, and thesolder is positioned in through-holes 2 over the contact lands 10 (notshown), which will each receive a ball 8 of transferred solder uponreflow. In FIG. 3E reflow has occurred. Because of the non-wettabilityof the material selected for the mold 1 and the chamfers 12 (not shown),surface tension of the molten solder 9 acts to contract the solder intoa ball shape 8. During reflow, a metallurgical bond is formed betweenthe solder ball 8 and the substrate land 10 (not shown). Therefore whencooling has occurred, the mold 1 can be lifted away without disturbingthe bonded solder balls 8, leaving behind an array of solder balls 8soldered to their respective lands 10 (not shown) in the completed PBGAsubstrate in FIG. 3F. If there will be a slight distortion of thesubstrate 6 and the mold 1 with respect to each other due to the soldertemperature, pressure can be applied after alignment during reflow, i.e.between 3D and 3E.

While the invention has been described in conjunction with specificnonlimiting embodiments, many modifications will be apparent to thoseskilled in the art in light of the foregoing information. Accordingly,it is intended that the present invention embrace all such modificationsas fall within the spirit and broad scope of the appended claims.

What is claimed is:
 1. An injection solder mold comprising a first majorsurface and a second major surface, the major surfaces penetrated andconnected by at least one double chamfered through-hole having a firstchamfered opening at the first major surface and a second chamferedopening at the second major surface, wherein the width of the at leastone through-hole is about equal to its height.
 2. The injection mold setforth in claim 1, wherein the material of which the mold is comprised isselected to be substantially non-wettable by the solder.
 3. Theinjection mold set forth in claim 1, wherein the material of which themold is comprised is selected to have a coefficient of thermal expansionsubstantially similar to that of a substrate onto which the solder willbe transferred.
 4. The injection mold set forth in claim 1, wherein thematerial of which the mold is comprised is graphite, FR4 resin laminateor a combination thereof.
 5. The injection mold set forth in claim 1,wherein the width is about 0.035 inches.
 6. The injection mold set forthin claim 1, wherein the aspect ratio of the at least one through-hole isbetween about 3:4 and about 1:2.
 7. The injection mold set forth inclaim 1, wherein a blind recess is provided in the second major surface.8. The injection solder mold set forth in claim 1 comprising also ablind recessed area to accommodate, with clearance, an IC chip.